Subquery returned more than 1 value

I need help debugging my query.

I get error:

Msg 512, Level 16, State 1, Line 5
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

I do not know how to begin testing to see where the error come from exactly.

Please help.

SELECT DISTINCT
		sqcl.SalesQuoteNumber
	,	sqcl.ItemNumber 
	,	(
			SELECT DISTINCT
				s.SalesQuoteNumber 
			FROM 
				cost.vStd_SalesQuoteCostLine s  
			WHERE 
				s.CustomerNumber_BillTo = sqcl.CustomerNumber_BillTo
			AND 
				s.ItemNumber = sqcl.ItemNumber
			AND 
				s.DocumentDate = 
								(	SELECT MAX(s2.DocumentDate)
									FROM cost.vStd_SalesQuoteCostLine s2
									WHERE s2.CustomerNumber_BillTo = sqcl.CustomerNumber_BillTo
									AND s2.ItemNumber = sqcl.ItemNumber
									AND s2.DocumentDate < sqcl.DocumentDate
								)
			) AS PriorQuoteNumber 
FROM 
	cost.vStd_SalesQuoteCostLine sqcl
WHERE 
	sqcl.SalesQuoteNumber = @SalesQuoteNumber

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